I may have solved with a slightly hacky solution, adding the blnLoading boolean (see below). I will test it for a bit before marking solved. Would be interesting to know that 'on activate for first time' event though:

Public WithEvents objInspectors As Outlook.Inspectors
Public WithEvents objMailInspector As Outlook.Inspector
Public blnLoading As Boolean




Private Sub Application_Startup()
    Set objInspectors = Outlook.Application.Inspectors
End Sub


Private Sub objInspectors_NewInspector(ByVal Inspector As Inspector)
    'Specify the message window
    If TypeOf Inspector.CurrentItem Is MailItem Then
       Set objMailInspector = Inspector
       blnLoading = True
       
    End If
End Sub


Private Sub objMailInspector_Activate()


    If blnLoading = True Then
        With objMailInspector
            .WindowState = olNormalWindow
            .Top = -1
            .Left = 4655
            .Width = 800
            .Height = 1041
        End With
        SendKeys "%7"
    End If
    blnLoading = False
    
End Sub